1. Field of the Invention
The present invention relates to pressure-activated cylinders, and more particularly to such cylinders known as gas springs, which utilize contained gas pressure to provide the lifting, lowering, moving and adjusting forces previously provided by the more complex and bulkier mechanical spring construction.
2. Description of the Prior Art
Since their introduction in the 1960's, gas springs have become recognized by industrial designers world-wide as an effective design alternative in many separate applications to the formerly used mechanical devices of greater complexity and cost. A gas spring essentially consists of a pressurized cylinder having a support rod extending therefrom. The support rod forms part of a reciprocating piston assembly, with an attached piston, or, more accurately, guide head remaining within the pressurized cylinder. A gas seal is formed at one end of the cylinder around the support rod, permitting the support rod to follow the guide head movement within the cylinder while preventing loss of pressurization. During operation, the pressurization within the cylinder acts upon the cross-sectional area of the support rod, forcing the guide head against the sealed end of the cylinder, fully extending the support rod from the cylinder. Retraction of the rod will thereafter occur upon application of a sufficient force against the rod to overcome the cylinder pressurization force. The rates of extension and retraction can be regulated by the design of the guide head and the manner of head engagement with the inner cylinder walls.
The speed and force applied by the advancing piston assembly can also be controlled by the selective location of the points for attaching the gas spring to the moveable structure, thereby generating specific leverage and/or moment arms to interact with the force supplied by the gas spring. A wide range of applied forces can thereby be obtained from a single, interchangeable model of gas spring. The compact nature of the gas spring along with the relative mechanical simplicity of achieving a specific applied force has led to the wide-spread use of gas springs in a variety of applications. Gas springs are commonly found on passenger cars, raising and supporting both the trunk lid and the engine hood. Commercial vehicles such as buses and airlines often use them for opening and supporting door panels to baggage storage areas and for other types of maintenance access panels. Gas springs are also to be found in office photocopy machines, either for providing access to the mechanized parts or for supporting the paper feed unit. In all of these applications, the gas spring provides a compact and simple mechanism for the controlled lifting and supporting of structural weight.
Gas springs, like their mechanical counterparts, will eventually wear out. Failure of gas springs occurs when pressurization of the cylinder is lost or, as is more frequently the case, falls below the level necessary to support the moveable structure. Loss of pressurization will typically occur due to gas leakage from around the piston rod seal. Seal failure is a normal consequence of the wear placed upon the seal by the repeated in and out movements of the piston rod. This normal rate of wear, however, will be greatly accelerated when the gas spring is placed in a dirty environment. An abrasive coating is formed on the extended piston rod by dirt and other contaminants, which increases the normal wear placed on the seal when the piston rod again retracts into the cylinder. However, in either case, movement of the piston rod will eventually abrade the seal sufficiently to permit the gradual escape of the pressurizing gas.
In an attempt to prolong the life of gas springs, manufacturers have selected charging gases of heavier molecular weights in order to reduce the rate of diffusion from the cylinder. However, the manufacturers must also remain concerned about the toxicity of gas employed, and the increased material costs associated with the heavier gases. Attempts have also been made to improve the mechanical seal surrounding the piston arm.
The foregoing improvements have, to some extent, extended the life of gas springs. However, gas leakage around the seal remains an inevitable, ultimate cause of their failure. The concept of refilling the cylinders to replace lost gas has almost uniformly been rejected by the manufacturers, since the labor costs of performing the refilling operation would exceed the cost of replacing the gas spring. Moreover, the rate of gas leakage will increase over the life of the gas spring as the seal wears out, and a refilled cylinder would have a reduced operating life-necessity another repair or replacement operation and addition labor costs.